Figure 8

Illustration of the significance of α₁Arg141 and α₂Val1 in Hb allostery. The close correlations of α₁Arg141, α₁Tyr140, α₁His87 and other crucial residues in the Hb allosteric transition are illustrated. The active area between the two α subunits is highlighted with a blue circle in the Hb crystal structure at the left bottom. All “inter-subunit” salt bridges are designated as red dash lines, while all “intra-subunit” salt bridges are expressed as green dash lines. The covalent bonds are specified as black solid lines. Color code: residues participated in the inter-subunit salt bridges: red; residues involved in the intra-subunit salt bridges: green; the proximal histidine and α-heme group: orange. Note the close association between α₁Arg141 and the proximal histidine, α₁His87 (and therefore the α-heme group), as well as the essential role of α₁Arg141 in constituting the α₁/α₂ interface by forming the T state stabilizing α₁Arg141–α₂Asp126 and α₁Arg141–α₂Lys127 inter-subunit salt bridges (and their symmetric counterparts, α₂Arg141–α₁Asp126 and α₂Arg141–α₁Lys127). α₂Val1 is also closely correlated with the α₁/α₂ interface through the α₂Val1–α₂Lys127 intra-subunit salt bridge. The close associations of αArg141 with two previously identified allosterically-crucial salt bridges, α₁Asp94–β₂Trp37 (the hinge point) and α₁Tyr42–β₂Lys99 (the switching region) are also revealed.